Semiconductor storage devices are used in integrated circuits for electronic applications, including radios, televisions, cell phones, and personal computing devices. Commonly known storage devices include charge-storing devices such as dynamic random access memories (DRAMs) and flash memories.
A more recent development in storage devices involves spin electronics, which combine semiconductor technology and magnetic materials. The spin polarization of electrons, rather than the charge of the electrons, is used to indicate the state of “1” or “0.” One such spin electronic device is a spin torque transfer (STT) magnetic tunneling junction (MTJ) device 10, as shown in FIG. 1.
MTJ device 10 includes free layer 12, tunnel layer 14, and pinned layer 16. The magnetization direction of free layer 12 can be reversed by applying a current through tunnel layer 14, which causes the injected polarized electrons within free layer 12 to exert spin torques on the magnetization of free layer 12. Pinned layer 16 has a fixed magnetization direction. When current I1 flows in the direction from free layer 12 to pinned layer 16, electrons flow in a reverse direction, that is, from pinned layer 16 to free layer 12. The electrons are polarized to the same magnetization direction of pinned layer 16 after passing pinned layer 16, flowing through tunnel layer 14, and then into and accumulating in free layer 12. Eventually, the magnetization of free layer 12 is parallel to that of pinned layer 16, and MTJ device 10 will be at a low resistance state. The electron injection caused by current I1 is referred to as a major injection.
When current I2 flowing from pinned layer 16 to free layer 12 is applied, electrons flow in the direction from free layer 12 to pinned layer 16. The electrons having the same polarization as the magnetization direction of pinned layer 16 are able to flow through tunnel layer 14 and into pinned layer 16. Conversely, electrons with a polarization differing from the magnetization of pinned layer 16 will be reflected (blocked) by pinned layer 16, and will accumulate in free layer 12. Eventually, magnetization of free layer 12 becomes anti-parallel to that of pinned layer 16, and MTJ device 10 will be at a high-resistance state. The respective electron injection caused by current I2 is referred to as a minor injection.
To eliminate the parasitic loading of MRAM cells, when MRAM cells are integrated in MRAM arrays, word line selectors are used to electrically isolate unselected MRAM cells, on which no operations are to be performed, from source lines. For example, FIG. 2 illustrates an MRAM cell including MTJ device 10 connected to word line selector 20, which is controlled by word line 22. When MTJ device 10 is selected for writing or reading, word line 22 is set to logic high, so that writing/reading current I can pass MTJ device 10. For the unselected rows, word line 22 is applied with a logic low voltage. The addition of word line selector 20, however, limits the current that can flow through MTJ device 10. The writing current I of MTJ device 10 is limited by the current-providing capacity of word line selector 20. To effectively and reliably program MTJ device 10, writing current I needs to be very high. However, this requires word line selector 20 to be large. The increase in the size of word line selector 20 causes an increase in the required chip area usage by the MRAM cell. The requirement to increase the size of word line selector 20 conflicts with the demand of increasing the density of MRAM arrays. A solution is thus needed.